In recent years, a type of mobile communications network known as an “ad-hoc multi-hopping” network has been developed. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. As can be appreciated by one skilled in the art, network nodes transmit and receive data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access fixed networks and thus communicate with fixed nodes, such as those on the public switched telephone network (PSTN), and on other networks such as the Internet. Details of these advanced types of ad-hoc networks are described in U.S. Pat. No. 7,072,650 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, granted on Jul. 4, 2006, in U.S. Pat. No. 6,807,165 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, and in U.S. Pat. No. 6,873,839 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, the entire content of each being incorporated herein by reference.
As can be appreciated by one skilled in the art, the time of flight (TOF) is the time interval during which a radio signal travels from one wireless terminal to another wireless terminal. Multiplying the TOF with the speed of radio wave propagation provides the distance or “range” between the two terminals. The range itself is used in several applications. Since the range provides only a measurement of proximity, one range alone is very seldom used in applications. For computing the position of a mobile node, most of the applications use either a set of ranges between several terminals, or one range and additional information about the direction.
As known in the art, the TOF is computed using two-way or one-way ranging. In two-way ranging, the station that needs the range transmits a request for range evaluation to a reference station. The reference station answers the request instantly or after a known delay. The time between transmitting the request and receiving the response at the first station is two times the TOF since in two-way ranging, the station seeking the range initiates the dialog.
In one-way ranging, clock synchronization of all references (i.e., terminals or nodes) is required all the time. If the clock of a mobile terminal involved in the ranging also is synchronized with the references, the measured ranges are true, otherwise they are all affected by the same unknown error and are called pseudo-ranges. Although one-way ranging is a more bandwidth efficient method than two-way ranging, the process of synchronizing references does require two-way exchange of messages, thereby consuming bandwidth.
For computing the position of a wireless mobile terminal in a wireless network of n fixed references with the two-way ranging method, 2n ranging messages are transmitted, while with one-way ranging, only n+1 messages are transmitted, and from time to time, another 2n messages are transmitted for maintaining the synchronization between the clocks of the terminals.
A common example of one-way ranging is the operations performed by a global positioning system (GPS). That is, GPS is based on a method that uses a one-way ranging method. All GPS satellites have high precision synchronized clocks and geocentric ephemeredes. The mobile receives simultaneously signals from several satellites located above the horizon and computes its geocentric position. Although it is very simple and direct, the method can be used only in places where several satellites can be seen simultaneously. Very accurate precision of location can be achieved only with very sophisticated GPS equipment that remains immobile for some long time and can receive signals from a large number satellites. On narrow streets with tall buildings, inside buildings or underground, the method cannot be used because the number of received signals from satellites is not satisfactory.
Also, to comply with E911 requirements, several methods for locating wireless telephones have been developed. Because all Base Stations providing connectivity to mobile telephones can have clocks synchronized to GPS satellites with 10−11 second precision, the time difference of arrival (TDOA) became the most preferred method. According to the TDOA method, when the mobile transmits a signal, at least three synchronized base stations receive the signal and record the time of arrival (TOA) of the signal. Then these base stations transmit the TOA of the mobile signal to a central computing unit usually through a wired connection. The computing unit, which knows the geocentric position of each base station, computes the geocentric position of the mobile using the TDOA of signals by solving the intersection of hyperbolas. The main advantage of the method is that no change has to be made to the mobile telephone for enabling the operation. The drawback of the method consists in potentially limited propagation of the signal between the mobile phone to at least three base stations when the mobile is located on narrow streets with tall buildings, inside large buildings or underground.
The following documents, which are incorporated by reference herein, describe different types of one-way and two-way ranging system: U.S. Pat. Nos. 6,728,545, 6,453,168, 6,266,014, 6,208,297, 5,321,668, 4,357,609, 4,161,734, 4,161,730, 3,943,509 and 3,934,251. However, none of the techniques described in these patent employ unsynchronized terminal clocks for performing one-way ranging of terminals.
Accordingly, a need exists for an efficient system and method for performing one-way ranging of terminals in a wireless communications network, to avoid the use of high quality oscillators to achieve clock synchronization between terminals.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.